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The TE₂₀ high-order mode operation characteristics of the ridge-loaded (RL) and groove-loaded (GL) folded waveguide slow wave structures (FWG SWSs) with two electron beam channels are studied for high-power traveling wave tubes (TWTs) at the terahertz (THz) frequency regime. Operating in a higher order mode means that the SWSs can have larger structure dimensions to reduce the fabrication difficulty at the THz frequency band and use higher current to increase the output power. For FWG SWSs operating in the high-order mode, it is difficult to suppress backward wave oscillation (BWO) of fundamental mode. In this article, we proposed the RL and GL FWG SWSs to suppress BWO and studied the dispersion, coupling impedance, and transmission loss of them. The results show that the RL structure can substantially increase the start oscillation length of the interaction circuit and avoid using many circuit sections for spurious mode depression which ease the assembly process of the tube. The GL structure can broaden the bandwidth with high-order mode operation but with the ease of BWO of fundamental mode. The straight RL and GL waveguides are used to explain the above results, and the effect of electron beam channel position on coupling impedance is also studied.

A low-loss all-straight optical fiber vector magnetic field sensor (OFVMS) based on magnetic fluid (MF) is designed and proposed. The sensing structure is composed of a Section of the C-type optical fiber (CTF) embedded in a pair of multi-mode optical fibers (MMF) and wrapped in a capillary with MF. The asymmetry of the CTF provides support for the vector magnetic field measurement whose direction is perpendicular to the optical transmission direction. According to the results of experiments, it can be obtained that the refractive index (RI) sensitivity of the sensing structure is as high as −15716.6072 nm/RIU. In addition, the intensity sensitivity of the OFVMS in the calibrated 90° direction can reach 202.23 pm/G between 0 and 123 G. At the same time, the transmission loss of the proposed sensor structure is not increased by the filling of MF. The sensor demonstrated has dominance in low cost, easy manufacture, and lower loss compared with the same type of sensor device, which has application prospects in vector magnetic field sensing.

The temperature-dependent polarization induced bias drift is a major factor affecting the long-term zero-bias stability of integrated optic gyro (RIOG). The silicon nitride (Si3N4) with high-aspect-ratio core geometry allows high polarization extinction ratio and ultra-low propagation loss. We start from suppressing the polarization noise of the RIOG and design a single polarization Si3N4 waveguide ring resonator (WRR) with a radius of 17.5 mm. We established the polarization model of the WRR. It is found through simulation that when the polarization-dependent loss of the Si3N4 waveguide reaches 800 dB/m, it can maintain the single-polarization performance, thereby suppressing the polarization noise of the gyroscope. And the relationship between the coupling coefficient of the coupler and the limit sensitivity, finesse and transmittance was determined. The cross-sectional size of the Si3N4 WRR is determined to be $4\,\,\mu \text{m}\,\,\times 40$ nm simultaneously by simulation. Finally, through its processing and testing, a single-polarization WRR with a finesse of 127, the quality factor is $1.31\times 10 ^{7}$ , a transmission loss of 2.5 dB/m, and a polarization-dependent loss of 810 dB/m is obtained. Meanwhile, we prove its single-polarization performance at different temperature. The successful design and fabrication of the WRR is expected to achieve high precision RIOG.

A terahertz broadband polarization converter based on the metasurface of a double-split ring resonator is proposed. The structural parameters of this device have been optimized in numerical simulations. The results show that in the frequency range from 0.49 to 1.88 THz (bandwidth of 1.39 THz) the proposed device rotates incident linearly polarized waves through 90°. The polarization conversion rate is more than 80%. The coefficient of reflection for cross polarization is greater than 90% with a transmission loss of 1 dB. The surface current distribution of this structure was simulated and analyzed at three frequencies that give high conversion rates of the polarization. The mechanism underlying this high conversion rate is presented, and the dependence of the conversion rate on incident angle and polarization angle is stimulated and analyzed. The results show that the conversion performance of this device is good for incidence angles ranging from 0 to 30° and polarization angles ranging from − 10 to 10°. Compared with previous designs, the polarization converter has a simple structure and a broad operational bandwidth. It has potential applications in the field of terahertz polarization modulation.

In the sub-millimeter-wave band, the wave is transmitted between two aperture antennas merely in the near-field region because of high working frequency and large space transmission loss. The antenna gain defined in the far-field region is no longer applicable under certain circumstance. Therefore, the near-field gain, one of the most important parameters of the sub-millimeter-wave antenna, should be defined. Based on the transmission equation and near-field transmission efficiency formulation, the near-field gain expression for the aperture antenna is proposed. The proposed expression can be applied to the entire radiation area, including not only the near field but also the far field. From the perspective of the theoretical derivation and numerical simulation, the results of the near-field gain for the aperture antenna are obtained in the sub-millimeter-wave band. Through a comparison with the published research, it can be concluded that the proposed expression is reasonable, indicating the theoretical establishment of the near-field gain and its potential application to the sub-millimeter-wave antenna design and sub-millimeter-wave radar theory.

Modern optical fibers are capable of transmitting multiple wavelengths simultaneously using the wavelength division multiplexing (WDM) method for multiple–input multiple–output (MIMO) communication systems. They have been put to practical use by increasing the communication speed. This letter proposes a new WDM circuit that transmits and separates the TE $_{11x}$ , TE $_{11y}$ , and TM01 modes (polarization and multimodes) within a circular waveguide in the 5.6-GHz band. The transmission loss of each mode/polarization is up to 1.1 dB, and the isolation between modes/polarizations is more than 28.7 dB in a 200-mm-long circular waveguide. As a result, the independence of each mode and polarization applicable to MIMO communication systems is verified.

In this article, a 1-THz traveling-wave tube (TWT) based on flat-roofed sine waveguide (SWG) slow wave structure (SWS) has been studied theoretically and experimentally, and parts of the component have been fabricated. First, the flat-roofed SWG SWS has been optimized for working at 1 THz, the normalized phase velocity is around 0.287, and the interaction impedance is over $3.5~\Omega $ in the frequency range of 1.02–1.04 THz. Meanwhile, the performance of the designed TWT has been simulated, whose saturated output power is 720 mW and the corresponding gain is 28.57 dB. What is more, to decrease the transmission loss of the high-frequency structure, the input and output structures have been designed and a transition structure that transforms the waveguide to standard rectangular waveguide has been proposed. The simulation results show that the reflection coefficient of the novel proposed transmission system is less than −20 dB, and the insertion losses of the transition structure with the input and output waveguides are 2.45 and 1.68 dB, respectively. Finally, the designed SWS has been fabricated by deep reactive-ion etching (DRIE) and tested; ${S}_{{11}}$ is generally less than −10 dB in the frequency range of 1020–1040 GHz.

This paper presents an experimental investigation of an acoustic barrier composed out of an acousticmetamaterial unit cell. The design of the unit cell consists of a square prism, acting as a resonator,within a hexagonal recess manufactured out of a single material. Two materials were used tomanufacture the unit cell: PolyMide Polycarbonate and Polylite Polylactic Acid. The acousticperformance of the unit cell was quantified for both materials using the acoustic absorptioncoefficient and acoustic transmission loss values for frequencies between 100 Hz and 5,000Hz. Theexperimental results indicate that the design reduced the peak absorption coefficient for bothmaterials while also introducing two additional peaks at around 1,500 Hz and at 4,000 Hz. Changesto the absorption coefficient values were observed for frequencies above 1,000 Hz while minimalchanges were observed for frequencies below 1,000 Hz. These results indicate that the proposeddesign, is able to widen the effective frequency band, or stop band for acoustic absorption forfrequencies above 1,000 Hz compared to the absorption coefficient of the material. The experimentalresults also indicate that the design increases the peak transmission loss by about 7 dB at 4,000 Hz.For sounds below 3,000 Hz, the design will only change the transmission loss by about 3 dB forfrequencies between 100 Hz to 3,000 Hz. These results indicate that the acoustic metamaterial design,consisting of resonator in a recess manufactured out of a single material, is able to broaden theeffective frequency range for sound absorption for frequencies between 1,000 Hz and 4,000 Hz andat increasing the transmission loss values for frequencies between 3,000 Hz and 5,000 Hz. It can beconcluded that the resonator in recess metamaterial design, manufactured out of a single material,can be used to increase the stop band for acoustic absorption for frequencies above 1,000 Hz and toincrease the transmission loss for frequencies above 3,000 Hz.

This study proposes a refractive index sensor based on tilted fiber Bragg gratings (TFBGs) that connect metal-reflective spherical ends. The proposed fabrication method enables it to reflect light to increase the overall energy spectrum. We used the end of the fiber with a spherical shape and the coating reflective metal to compare the reflective energy in TFBGs. The reflective technique was employed to deposit nickel (Ni) and silver (Ag) onto the surface of the spherical end of TFBGs to precisely form the spherical-shaped tilted fiber Bragg gratings (STFBGs). The Ni-covered NiSTFBG sensor had an average sensitivity of 8.028 nm/RIU wavelength, which is a 1.64-fold improveme nt. The result shows the backward reflected wave is reflected by the metal layer has improved performance for the wavelength and transmission loss via the reflected metal spherical-shaped end of tilted fiber Bragg gratings.

A wideband terahertz (THz) output network consisting of a pyramidal horn and a diamond Brewster window has been developed for 0.7–1.0-THz traveling-wave tube (TWT) amplifier and is measured at 0.325–0.5 THz owing to the limitation of the measurement facility. A pyramidal horn is designed to convert the waveguide mode into a high-purity quasi-plane polarized mode to meet the demand of the Brewster window. A diamond Brewster window plate with low THz transmission loss is fabricated via chemical vapor deposition (CVD). The plate is directly placed on the pyramidal horn with a Brewster angle. A circular window plate supporter is intended to balance the stress distribution during diamond-metal brazing. The measurement shows transmission coefficient is better than 0.85 within a wide bandwidth of 0.325–0.5 THz. This letter provides a practical way for a single diamond window structure for THz TWT amplifiers.

This letter presents transmission enhancement methods for low-emissivity (low-E) windows at millimeter wavelength (mmWave) frequencies. These methods are required for installation of effective indoor customer premises equipment for mmWave fixed wireless access (FWA) services. Low-pass filtering frequency selective surfaces are patterned with low-E coatings by laser, and the effects are verified theoretically and experimentally. Antireflection (AR) dielectrics are additionally utilized and found to reduce transmission loss in various types of window regardless of frequency and incident angle. The results of the current study achieve lower losses for various types of window and incident angles. A transmission loss improvement is also achieved in a real FWA scenario, which can be applied for the coverage extension of FWA services.

In this paper, we numerically study a curved frequency selective surface (FSS) radome, which encloses a waveguide slot array antenna operating at 10GHz, using the ray tracing technique and flat model. The transmission loss (TL) and boresight error (BSE) are calculated for various degrees of freedom, such as elevation and azimuth angular scanning or gimbal systems of array antennas to examine the electromagnetic properties of the curved FSS radome. Our calculations of radiation patterns, TL, and BSE for a multi-layered dielectric radome are compared with those of a commercial EM solver for validation. Importantly, we quantify phase distortions, incurred by the curved FSS radome, using the insertion phase delay (IPD) of transmitted fields on the radome surface. Thereby, we demonstrate that the BSEs strongly depend on (1) the spatial distribution of phase distortions on the radome surface and (2) their average level. The present method is highly suited for analyzing radomes with arbitrary surface patterns inserted.

In this article, a transmission line system for the propagation of millimeter-wave radiation is presented. The full system includes a TE11-to-TE01 mode converter, waveguide tapers, miter bends, and many straight sections. The design of each of these components is described, and the optimized simulation results are given. The mode converter shows a greater than 96% mode conversion efficiency that can be achieved over a 2% bandwidth at the ${W}$ -band. The miter bends demonstrate a transmission loss of 0.04 dB each over the same bandwidth when they are configured to introduce a mixture of higher order waveguide modes before the reflecting surface. An example transmission line system with a propagation length of 20 m, inclusive of four 90° bends with an oxygen-free high conductivity (OFHC) copper waveguide material, was studied over a 90–96-GHz frequency range and showed a 0.84-dB transmission loss at 93 GHz.

We propose a hole-assisted solid core Bragg fiber (SCBF) with a high-index-contrast cladding for near-infrared broadband single-polarization transmission in this article. The circular symmetry of a common SCBF is broken by the introduced air holes in the solid core for increasing the modal birefringence and polarization-dependent loss (PDL). In the proposed single-polarization SCBF, the confinement of guided modes predominantly depends on the photonic bandgap effect from the high-index-contrast cladding, and simultaneously rests with the total internal reflection from the interface between the solid core and air holes. The numerical results show that both the enhanced modal birefringence and PDL are strongly related to the refractive index, size and locations of air holes. We demonstrate that the modal birefringence for the fundamental mode can be up to the order of 10−3, and the difference in confinement losses between both polarized fundamental modes is no less than an order of magnitude, which is sufficiently large to support single-polarization operation. Moreover, the single-polarization bandwidth with the transmission loss lower than 0.1 dB/m is as high as 277 nm and the critical bend radius is as low as 1 cm, indicating the excellent performances of high bandwidth and bending resistance in the hole-assisted single-polarization SCBF.

In this letter, a novel waveguide-below-cutoff (WBC) structure with electromagnetic bandgap (EBG) surfaces is proposed to increase the cutoff frequency without sacrificing the waveguide size. To implement the proposed WBC structure, EBG arrays with various via patterns are fabricated and attached to the wide walls inside a normal rectangular waveguide. Transmission loss of the WBC structure is measured in terms of different EBG arrays, and it is confirmed that the cutoff frequency could be extended to higher frequencies accordingly.

A novel Ka-band azimuthal supported angular log-periodic strip-line (ASALS) slow wave structure (SWS) is proposed for a miniature radial sheet beam traveling wave tube (TWT). In the proposed SWS, two dielectric rods are employed to support the ASALS at both azimuthal sides. The strip-line is made of high-melting metal; thus, the ASALS SWS has a higher power capacity than microstrip SWSs. The high-frequency characteristics of that proposed SWS are investigated by using simulation software. The ASALS SWS is then fabricated and assembled for cold test. The measured transmission loss and reflection loss are better than − 2.5 dB and − 10 dB, respectively. The hot performance of the proposed is studied based on particle-in-cell (PIC) simulation. It is shown that when the operating voltage is 5700 V, the ASALS TWT can give a maximum output power of 320 W at the frequency of 39 GHz, corresponding to a gain of 14.9 dB and electron efficiency of 9.12%.

With the increased plasma frequencies, millimeter wave diagnose of plasma over a broadband range is technically preferred. This article describes the design, fabrication, and measurement of a compact triband quasi-optical system covering 33–110 GHz for the EAST tokamak plasma detection. The overall size of the quasi-optical system is 300 mm by 300 mm. Two broadband frequency selective surfaces are implemented with patch-grid structure to separate the signal to the $Q$ -band, $V$ -band, and $W$ -band. A trade-off is made between the selectivity and bandwidth. Horn antennas are designed based on a multiflare structure to produce good Gaussian beam radiation. The measured results agree well with the simulated ones. The overall transmission loss of the whole system is better than 3 dB in the $Q$ -band and $V$ -band, and it is better than 7 dB in the $W$ -band. The system has been deployed on EAST tokamak for real-time plasma detection.

The airborne radomes have to cater superior electromagnetic (EM) performance with bandpass characteristics of stealth application. In this regard, a hybrid A-sandwich radome is proposed in this paper. The proposed radome consists of a novel strongly coupled frequency selective surface (FSS) core sandwiched between two dielectric layers (acts as skin) to form an A-sandwich structure. The dielectric layers are cascaded in such a way that the middle layer has less dielectric parameters than the skin dielectric. The core layer comprises a modified FSS array using strongly coupled FSS layers through a series of metallic vias. This strongly-coupled FSS element will have the advantage of eliminating inter-element interference and improves the EM performance characteristics of the structure. The structure exhibits very good band-pass characteristics (>90%) at a normal impinging angle with sharp roll-off characteristics. To show the efficacy of the proposed structure, the transmission loss has been compared with that of conventional A-sandwich radomes at 0°, 50° incidence angle for both TE and TM polarization. Conformal analysis of the unit cell has been carried out, and sector-wise thickness optimization was performed to analyze the structure for the conformal shaped radome application. Finally, a physical prototype has been fabricated and measured its scattering parameters, radiation characteristics in a fully shielded anechoic chamber. The results are encouraging and prove its suitability for radome application.

We propose and numerically analyze various hollow-core antiresonant fiber (HC-ARF) for operation at terahertz frequencies. We compare typical HC-ARF designs with nested and adjacent nested designs while analyzing performance in terms of loss and single-mode guidance of terahertz waves. With optimized fiber dimensions, the fundamental core mode, cladding mode, core higher-order modes (HOMs), and the angle dependence of adjacent tubes are analyzed to find the best design for low loss terahertz transmission. Analysis of the fiber designs shows that the nested tube-based antiresonant fiber exhibits lower transmission loss and superior HOM suppression, exceeding 140. The nested HC-ARF is feasible for fabrication using existing fabrication technologies and opening up the possibility of efficient transmission of terahertz waves.

The long-range propagation modes in an acoustic channel under ice are basically caused by supercritical incidence. The energy distribution and transmission loss in the acoustic channel under ice are changed by a scatter in ice. The influence of a slender cylindrical cavity near and parallel to the ice-water interface on the sound propagation is analyzed using Fourier-Bessel series and Sommerfeld-Watson transformation. The research found that the acoustic field presents a beam in the mirror reflection direction at supercritical incidence, and the beam-width is proportional to secant of incident angle; meanwhile, the reflected coefficient is proportional to cosine of incident angle. The reflection coefficient increases with relative depth and Helmholtz number if the incident angle is a constant.

The sine waveguide (SWG) is presented as a potential slow-wave structure (SWS) for the millimeter-wave (mm-wave) and terahertz (THz) traveling-wave tube (TWT) because of the advantages of wide bandwidth, natural electron beam tunnel, and easy fabrication. In particular, this theoretical study indicates that the transmission loss of the SWG is far less than that of the folded waveguide SWS, which is widely employed in the mm-wave and THz TWT. Here, the flat-roofed SWG slow-wave circuit working in the ${W}$ -band has been machined using nano-computer numerical control technology, which guarantees micron tolerances and surface roughness of tens of nanometers using carbide tooling with 100- ${\mu }\text{m}$ diameter. The cold test results show that the transmission coefficient ${S}_{{{21}}}$ of the entire high-frequency slow-wave circuit is more than −4.1 dB with a total length of 123.84 mm in the frequency range from 90 to 100 GHz, and the transmission loss is less than 0.36 dB/cm.

Slow-light performance based on photonic-crystal coupled-cavity waveguide (PC-CCW) is investigated with low-dispersion pulse transmission. A chain of periodically arranged specific defect cavities of air holes are created along the axis of the waveguide and each of the cavities looks like an irregular hexagon with arrow-like left and right sides to form the PC-CCW. By changing the radius and position of certain defects, slow light modes are achieved. Through optimization, we obtained a highest compromise value for an operating bandwidth of 12.9 nm with an average group index of 70.75 and a normalized delay-bandwidth product (NDBP) of 0.5873. This value of NDBP with low group velocity dispersion is higher than that in other structures based on waveguides or coupled cavities reported previously. The arrow-like shape of the cavity sides is shown to be perfect for reducing the transmission loss of the waveguide. The proposed structure with high NDBP is beneficial for buffering applications and slow light devices.

The proposed study contains the design and analysis of TM 11 to HE 11 (hybrid) mode converter at 42±0.2 GHz which is an integral component of the transmission line that transmits 200 kW power from gyrotron to the plasma in SST-1 Aditya tokamak. HE 11 mode is preferred, as the transmission loss in corrugated waveguide is less in comparison with other modes. For generating HE 11 mode from TM 11 mode, the depth of the annular slots in the mode converter is varied gradually from zero to one-quarter wavelength. The conversion efficiency and radiation pattern of the proposed mode converter depends on the corrugation profile and the length of the mode converter. Both the parameters are optimised numerically to achieve the highest conversion efficiency. Keeping in mind mechanical fabrication of mode converters optimisation has been carried out. It converts 95.6% power of TM 11 mode into HE 11 (82.2% TE 11 +13.4% TM 11) mode respectively. The 1 dB compression bandwidth of 19.04% (8 GHz) at 42 GHz is obtained. The length of mode converter is 40.83 λ mm. The reflection coefficient and VSWR are -25 dB and 1.1 respectively. The power in side lobes and isolation from cross polar component is less than -20 dB and -25 dB respectively.

This work presents a novel tunable first-order absorptive balanced filter. It consists of a coupled-line bandpass filter with absorption networks connected at its input and output ports. The absorption network is designed using semi-lumped elements. With the proposed architecture, reflectionless performance can be achieved for both the common-mode (CM) noise and out-of-band differential-mode (DM) signal. Besides, the circuit is compact by nature due to the use of semi-lumped elements. To demonstrate the proposed idea, a microstrip prototype is fabricated and measured. The measured results show excellent reflectionless performance for both CM noise and DM signal. The measured DM transmission loss varies from 3.7dB to 2.2dB within a frequency tuning range of 1.6GHz-1.9GHz. Meanwhile, the DM reflection coefficient is lower than −9.5dB from 0.1 GHz-5.6GHz. The CM reflection coefficient is lower than −10dB while the circuit still exhibits suppression higher than 22dB for the transmission of CM noise from 0.1GHz-2.3GHz.

In this paper, we propose free space optical (FSO) communication using laser light in the 2μm wavelength. FSO using the mid-infrared 2μm band can reduce the effects of turbulence and particles generated in free space and improve link performance. In addition, it has a large allowable exposure because it can be operated in a transparent window in the atmosphere or in a safety for human eyes. We will analysis link visibility, transmission characteristics and SNR of 2μm FSO, demonstrate the BER of the FSO link with fog experimentally. The theoretical analysis will show that visibility and transmission loss will be improved by 2μm FSO, and experimental results also show that BER performance can be improved about 2dB.

Prediction of loss or loss estimation is essential for estimating the performance of the circuits in the millimeter waveband. As an example, this paper shows the prediction of the transmission loss of the LTCC post-wall numerically and experimentally at 60 GHz band. The measured uncertainties of the dielectric properties of the LTCC have been considered in the loss estimation. The transmission loss of the waveguide has been quantitatively predicted with the associated confidence intervals in the millimeter wave.

Abnormal reflection is one of the most basic functions of metasurface and its efficiency is very important in practical application. The implementation of perfect anomalous reflection needs to meet both phase and amplitude requirements. However, due to the absorption loss of metal materials and transmission loss of dielectric materials, perfect abnormal reflection is difficult to be realized in optical frequency. Here, we propose a multi-layer all-dielectric metasurface structure, using the reflecting layer to reflect all of this energy back to meet the amplitude requirement of 100% reflectivity. As proof of this concept, a 30° perfect anomalous reflector at 1550 nm for TM polarized light is showed. It is worth noting that this approach is not limited to anomalous reflection but can be extended to more complex metasurface functions such as focusing.

In this paper, a high sensitivity compact multi-slot sub-wavelength Bragg grating refractive index (RI) sensor was investigated. The structural parameters were optimized for higher sensitivity to RI change of the surrounding medium from viewpoints of a wavelength shift, an extinction ratio and a transmission loss, and a record-high sensitivity was experimentally demonstrated with a compact size. In this sensor, the first side-lobe at the Bragg grating (BG) stop-band end was focused as a sensing peak wavelength for moderate transmission loss and efficient sensing. To realize the compactness, a period count of the BG was kept as small as 20. By increasing the RI of the surrounding medium, the sensing peak shifts toward a longer wavelength side; thus due to the high sharpness and easy tracing of the first side-lobe, the device worked as an efficient RI sensor. The structural optimization was carried out by using 3D finite-difference time-domain (FDTD) simulation approach, and also influences of the structural parameters to sensitivities were discussed. Based on these optimized parameters, the devices were fabricated using the lift-off technique. By exposing the sensor to various liquid samples with different RIs such as pure water, sugar-dissolved water with various concentrations, acetone and isopropyl alcohol (IPA), a record-high sensitivity of 730 nm/RIU was attained for a sensor fabricated on SOI platforms with a length of as small as 9.5 µm and a transmission loss of 3 dB.

In this paper, the transmission losses of grounded and undergrounded spoof surface plasmon polariton (SSPP) transmission lines (TLs) are investigated over a wide frequency range. To overcome the effects of transition structures of SSPP transmission lines, the multiline method is utilized for the de-embedding purpose. Attenuation constants of grounded SSPP (G-SSPP) lines, ungrounded SSPP (U-SSPP) lines and traditional microstrip (MS) lines in the same size are compared in detail. The impact of dielectric loss tangent on attenuation performance is also discussed.

A W-band hybrid-fed mictrostrip patch array for advanced real-time imaging applications has been proposed in the article. It is fed by a novel hybrid feeding network with lower loss compared with the parallel-fed network. The proposed array is composed of 16 series-fed taper microstrip patch subarrays with Chebyshev amplitude tapering. The series-fed subarray is fed symmetrically in the midpoint to achieve non-dispersive patterns at different frequencies. In order to decrease the transmission loss, a hybrid feeding network is de-signed to replace the multi-level parallel one between series-fed subarrays. The tapering distribution is determined by the width of the quarter wave impedance transformer in the hybrid feeding network. Simulation results show that the antenna array achieves an impedance bandwidth of greater than 6 GHz and a gain of greater than 26.8 dBi. The side-lobe level of the pattern at the center frequency is -22.5 dB and -22.4 dB in E-and H- planes, respectively.

In order to better understand the variation mechanism of ozone abundance in the middle atmosphere, the simultaneous monitoring of ozone and other minor molecular species, which are related to ozone depletion, is the most fundamental and critical method. A waveguide-type multiplexer was developed for the expansion of the observation frequency range of a millimeter-wave spectroradiometer, for the simultaneous observation of multiple molecular spectral lines. The proposed multiplexer contains a cascaded four-stage sideband-separating filter circuit. The waveguide circuit was designed based on electromagnetic analysis, and the pass frequency bands of Stages 1-4 were 243-251 GHz, 227-235 GHz, 197-205 GHz, and 181-189 GHz. The insertion and return losses of the multiplexer were measured using vector network analyzers, each observation band was well-defined, and the bandwidths were appropriately specified. Moreover, the receiver noise temperature and the image rejection ratio (IRR) using the superconducting mixer at 4 K were measured. As a result, the increase in receiver noise due to the multiplexer compared with that of only the mixer can be attributed to the transmission loss of the waveguide circuit in the multiplexer. The IRRs were higher than 25 dB at the center of each observation band. This indicates that a high and stable IRR performance can be achieved by the waveguide-type multiplexer for the separation of sideband signals., 29 pages, 18 figures, accepted for publication in Journal of Infrared, Millimeter and Terahertz Waves

A compact log-periodic antenna based on Spoof surface plasmon polaritons(SSPPs) is proposed in this work. The antenna consists of three part: the microstrip line, the SSPPs conversion part, and radiation part. The SSPPs structure generates surface waves, which are eventually radiated by dipoles acting as radiators. The SSPPs mode has high field limitations, and tightly restrain the signal in the groove structure of SSPPs to ensure the efficient transmission of the signal and reduce the transmission loss. The simulation results show that the voltage standing wave ratio(VSWR) of the antenna is lower than 2 and the gain is higher than 6.1 dB in the frequency range of 7.3-10.6 GHz. The proposed antenna has a good performance of low profile, compact structure, which is of great value in wireless communication systems.

High transmission efficiency metasurface unit cells have been designed based on surface electric and magnetic impedances derived from Huygens&rsquo, principle. However, unit cells for low transmission loss (&lt, 1 dB) over a wide transmission phase range require at least three metallic layers, which complicates the unit cell design process. In this paper, we introduce high-efficiency Huygens&rsquo, metasurface unit cell topologies in double-layer FR4 printed circuit board (PCB) by implementing surface electric and magnetic current using the top and bottom metallic patterns and via drills. Eleven unit cells were optimized for wide phase coverage (&minus, 150&deg, to 150&deg, ) with a low average transmission loss of &minus, 0.82 dB at 10 GHz. To demonstrate the high-efficiency of the designed unit cells, we designed and fabricated two focusing lenses with dimensions of near 150 &times, 150 mm (5&lambda, &times, 5&lambda, ) to focus a spherical beam radiated from short focal distances (f = 100 and 60 mm). The fabricated focusing lens showed 12.87 and 13.58 dB focusing gain for f = 100 and 60 mm at 10 GHz, respectively, with a 1 dB fractional gain bandwidth of near 10%. We expect that the proposed focusing lens based on high-efficiency double-layer metasurface unit cells can help realize compact and high-gain focusing lens-integrated antenna systems.

In this paper, a switchable circular polarizer based on varactor-loaded metasurface is proposed. With an incident wave of 45° linear polarization (LP) plane wave, the polarization of the transmission wave through this polarizer can be switched between left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) by controlling the bias voltage of the varactors. Simulation results show that the designed circular polarizer exhibits transmission loss better than −1 dB, and phase error within ±5° in frequency band 29.4 – 30.8 GHz.

Periodically slotted leaky elliptical waveguides filled with different cross section shape dielectrics are proposed and analyzed in this paper. Because the air filled leaky waveguide is easily being damaged, so dielectric is usually filled in waveguide to avoid the extrusion, which also causes additional transmission loss. The transmission properties of leaky waveguides with different cross section shapes of dielectric filing are simulated and analyzed. The results show that if the cross section shape of filling dielectric is properly designed, the elliptical dielectric-filled waveguide can realize relatively low transmission attenuation, and can be used in the application of wireless communication in long-distance confined space.

A generic fiber positioning strategy and a fabrication path are presented for microlens-fiber-coupled integral field units. It is assumed that microlens-produced micro-images are carried to the spectrograph input through step-index,multi-mode fiber, but our results apply to micro-pupil reimaging applications as well. Considered are the performance trades between the filling percentage of the fiber core with the micro-image versus throughput and observing efficiency.A merit function is defined as the product of the transmission efficiency and the etendue loss. For a hexagonal packing of spatial elements, the merit function has been found to be maximized to 94% of an ideal fiber IFU merit value (which has zero transmission loss and does not increase the etendue) with a microlens-fiber alignment (centering) tolerance of 1 um RMS. The maximum acceptable relative tilt between the fiber and the microlens face has been analyzed through optical modeling and found to be 0.3 degree RMS for input f-ratio slower than f/3.5 but it is much more relaxed for faster beams. Several options of fabricating fiber holders have been compared to identify cost-effective solutions that deliver the desired fiber positioning accuracy. Femto-second laser-drilling methods deliver holes arrayed on plates with a position and diameter accuracy of 1.5 um RMS, and with an aspect ratio of 1:10. A commercial vendor produces plates with thickness of 5 mm, but with similar (1 um RMS) positioning accuracy. Both of these techniques are found to be moderately expensive. A purely photo-lithographic technique performed at WCAM (a facility at the University of Wisconsin, Madison), in tandem with deep reactive ion etching, has been used to produce a repeatable recipe with 100% yield. Photo-lithography is more precise (0.5 um RMS) in terms of hole positioning and similar diameter accuracy (1 um RMS)., 26 Pages, 18 Figures, 1 Table

This paper discusses the temperature dependence of the complex permittivity in the millimeter-wave band of dielectric materials used in printed circuit boards and its transmission loss effect. In the structure of a newly developed balanced-type circular disk resonator, two dielectric material samples sandwiching a thin circular copper disk can expand and contract under uniform pressure against temperature changes, which means the frequency dependence of the complex permittivity between 10 GHz and 95 GHz can be stably measured in a temperature range from -30°C to 130°C. To confirm the validity of measured values, a printed circuit board having a transmission line with a microstrip structure was fabricated, and transmission loss was measured at room temperature. Then, the dielectric material and the copper foil were taken out, the frequency dependence was measured, and the transmission loss of the transmission line was calculated by HFSS (three-dimensional electromagnetic field analysis solver) using these measured values. In the present model, transmission loss value calculated using the measured value is 10% closer to the measured value than that using the catalog value, and almost 96% of the measured values could be fitted. The frequency dependence of other dielectric materials was measured between 10 GHz and 95 GHz at temperatures of -30°C, 25°C, and 130°C. A difference of up to 0.57 dB/cm was obtained between the transmission loss calculated from the complex permittivity of the catalog value for 10 GHz at the temperature of 25°C and the transmission loss calculated from the measured value, confirming the effect of the temperature dependence of complex permittivity.

Increasing applications of terahertz (THz) waves require design and fabrication of new waveguides with low transmission loss in THz region (0.1 ∼ 10 THz). Hollow-core photonic crystal fibers (HC-PCFs) with their unique characteristics hold great promise for efficient THz transmission. Hence, in this article, HC-PCFs are designed and simulated based on the finite-difference-time-domain (FDTD) method to efficiently transmit THz radiation. Impacts of structural parameters such as background material, core diameter, air-hole rings and filling factor on the THz transmission window of HC-PCFs are investigated. The results show that as the number of air-hole rings and the core diameter are increased the transmission loss is decreased. Also, the suitable HC-PCF in the transmission window of 1.55–1.85 THz with simultaneously good dispersion properties is obtained when the background material is Teflon.

This paper presents a first design of a circular ring frequency selective surface (FSS) with reflecting 140 GHz microwave signal (D-band) while transmitting through Q-band (33-50 GHz) and V-band (50-75 GHz) signals, which is used in a microwave system. Relationship between the size of the ring inner diameter and the resonance frequency of the FSS, and the influence of the microwave incident angle on the transmission loss at the resonance frequency (140 GHz) and the passbands (33-75 GHz), are surveyed. Result shows that the resonance frequency decreases with the ring inner diameter increases, which is consistent with the theoretical calculation results. Moreover, the maximum transmission losses at different resonance frequencies are all below −16 dB. For the candidate size, the transmission losses at the resonance frequency and the passbands are about −25 dB and −0.1 dB, and the deviation of the incident angle is acceptable within 8°.

Frequency selective surfaces (FSSs) are low-profile structures consisting of periodic elements that can manipulate the behavior of electromagnetic waves. In various fields, such as vehicular and indoor communication and personal electronic devices, their use as spatial filters is limited primarily due to safety and aesthetic reasons. A possible solution in this regard is using transparent conducting oxides or graphene on glass. However, the transmission performance of these filters is relatively poor as they inherently suffer from higher degrees of ohmic loss as compared to their opaque counterparts. Thus, we propose a suitable alternative that offers lower transmission loss with similar a transparency profile (76.2%) using copper mesh instead. The present letter discusses the design of an optically transparent triple-band bandpass filter that passes electromagnetic waves only at 2.4, 3.7, and 5.7 GHz and provides a minimum attenuation of 35 dB at transmission zeros. Investigations have also been carried out on the proposed structure under variations in both polarization ( $\alpha _{1}$ ) and incidence ( $\alpha _{2}$ ) angles. The designed FSS has wide angular stability and is fourfold symmetric. Thus, its filtering performance is largely unaffected under variations in polarization angle. A prototype of the proposed filter has been fabricated and experimentally tested for validating the simulation results, and a good agreement between them has been achieved.

A megawatt class X-band gyro-twystron using periodically loaded alternate lossy dielectric and metal output waveguide section is studied for its beam-wave interaction behavior using a fully nonlinear, self-consistent, and multimode theory. Nonlinear amplification of the desired TE01 mode and its competing parasites including TE11, TE21, and second harmonic TE02 modes are investigated using multimode code to ensure the device stability. The present amplifier developed a peak RF output of ~25 MW in the desired TE01 mode that is validated with a 3-D commercial particle-in-cell code. Furthermore, an input coupler and an output RF window are simulated for their transmission and reflection characteristics. The return and transmission losses of input coupler are calculated as ~28 and ~0.13 dB, respectively. Similarly, the RF window is designed to be transparent for the output with a transmission loss of ~0.075 dB and a return loss of ~63 dB.

A novel method to measure the reflectivity of fiber Bragg grating (FBG) is proposed and demonstrated experimentally. A kind of fiber Bragg grating FP(FBG-FP) cavity is constructed, which includes a couple of identical uniform FBG as the cavity mirrors linked by a piece of Er-doped fiber. The response of FBG-FP cavity to pulsed laser injection is demonstrated theoretically and experimentally, and the effect of the reflectivity of FBG to the output of FBG-FP cavity is discussed. The reflectivity of FBG is achieved by measuring the transmission loss of FBG-FP cavity with cavity ring-down spectroscopy technique. A general relationship between the reflectivity of FBG and ring-down time of FBG-FP cavity is derived. It’s demonstrated that the output loss of FBG-FP cavity increases as the reflectivity of FBG decreases, the stable light intensity in the cavity decreases fastly, and the ring-down time of the FBG-FP cavity decreases, the operating wavelength of FBG is a function of its temperature. The results demonstrate a new concept of the measurement of reflectivity of FBG and the technical feasibility.

A hollow-core fiber with nested anti-resonant node-free cladding tubes suitable for broadband THz guidance with low transmission losses is proposed. It is shown that the tube separation and tube thickness of the inner elements have a significant effect on the confinement loss and effective material loss of the fiber in the THz band. Using TOPAS copolymer, the proposed fiber was optimized for operation at 1 THz and it is predicted from numerical simulations that loss can be reduced to as low as 0.05 dB/m with a 0.6 THz wide dispersion flattened bandwidth. 426 477 482

In practical applications, the signal-to-noise ratio (SNR), reliability, and the long-term stability of wavelength modulation spectroscopy (WMS) system is limited by the non-absorption transmission loss. In this investigation, we report on a signal recovery method to remove the impact of non-absorption transmission loss based on an automatic gain closed-loop control. The deviation of the WMS system with the introduced signal recovery method has been observed to be below 4.27% under the non-absorption transmission loss which varied from 0 dB to −6.02 dB, and is about 13 times lower than the ordinary system. The proposed method is suitable for promoting the reliability and stability of WMS system under rigorous circumstances. Besides, it has the potential value in the multi-point detection or to achieve a higher SNR.

A single-layered triband transmissive/reflective frequency selective surface (FSS) with incident angle up to 80° and polarization stability is designed, simulated, and measured. The periodic element includes a compact hexagonal arrangement of Y-slot and Y-patch, which can effectively reduce unit spacing to 0.16 $\lambda $ . Compared with the conventional hexagonal arrangement, compact arranged Y-unit has a larger band range from 1 to 40 GHz using grating lobes suppression. Controllable passband in X -band and reflectband in Ku -band and Ka -band are proposed. Transmission loss is less than 1 dB in 9.41 GHz in bandwidth of 5% when the incident angle is from 0° to 80°, while the return loss in bandwidth of 5% when 14.5 and 35 GHz is less than about 1 dB when incident angle range is from 0° to 80°/60° for dual polarization, respectively. Hyperboloid FSS is simulated, and performance of transmission and reflection is good. To the best of our knowledge, this is the first report of single-layered FSS that simultaneously operates for angle of incidence more than 60° in X/Ku/Ka -bands for dual polarization.

This study explores the use of advanced optimization algorithms for determining optimum parameters for grating based sensors. Shift in wavelength in the resonance bands is always accompanied by reduction in loss of the attenuation peaks due to changing coupling coefficients in long period grating sensors. Maximization of strength of the rejection bands is considered as the objective function to enhance the range of detection for both amplitude and wavelength based demodulation schemes. Effect of grating period, length of the grating and peak induced index change, on core cladding mode resonances has been analyzed in detail. Transmission loss in resonance bands and shift in the resonance band are strong functions of the grating period, length of grating, peak induced-index change, order of the cladding mode and fiber parameters. The results of effect of optimization algorithms on the convergence and fitness values of the objective functions are reported. Transmission spectra obtained by analytical model has been verified with the experimental results. Maximum transmission loss of ≈76 dB has been achieved with optimized grating parameters. Effect of variation of the optimized parameters by 10% on the strength of rejection band has been analyzed.